Ideally, quantum chemistry methods such as multi-reference coupled cluster should be preferred for the task of computing atomic forces as they automatically include electronic correlation effects leading to dispersion forces as well as to correctly treating the magnetic degrees of freedom. However, their computational cost virtually limits their application to small gas-phase molecules. When it comes to periodic systems, Density Functional Theory (DFT) is the only option. Moreover, for large systems comprising several hundreds of atoms, as often is the case for molecular crystals, DFT in its Generalized Gradient Approximation (GGA) is often the best, if not unique, option. GGA functionals, like PBE[1]}, are generally recognized to provide a good estimation of bond lengths and vibrational force-constants. Alternatively, hybrid functionals also provide a good choice for the simulation of vibrational and thermodynamic properties of molecular systems[2]}, but the computational overheads often hinder their use for solid-state systems.
